Managers expected that a few people would take advantage of the offer and were shocked when almost 500 walked out.

One worker told the Times: “We were told we could go home if we couldn’t hack the heat – and we didn’t need a second invitation. It was much better being at home in the shade of the garden than slaving away on people’s tax returns.”

More than 3,000 people work at the tax offices but only one office block was affected by the air-conditioning failure.

A Revenue spokesman said: “In the interests of health and safety, management agreed to staff going home during the period of hot weather. Phone lines and customer advice services were not affected.”

Under the 1992 Workplace (Health, Safety and Welfare) Regulations, there is no legal maximum or minimum temperature requirement under which employers are obliged to operate.